Michael Martin, the Speaker of the House of Commons, was under growing pressure last night to explain his use of generous parliamentary expenses and allowances.

As the Conservatives warned that Martin had a "problem" - and Labour MPs claimed he was the subject of a "witch-hunt" - the speaker faced the threat of a parliamentary investigation into his finances.

John Lyon, the parliamentary commissioner for standards, was last night asked by the TaxPayers' Alliance to investigate whether Martin had abused parliamentary expenses and allowances in the light of a series of new disclosures. Lyon is obliged to examine all such complaints though the commissioner could rule that the complaint is unfounded.

Martin, who became speaker of the Commons in 2000 after the retirement of the popular Betty Boothroyd, found himself under pressure after disclosures over the weekend. These included:

· The resignation of his spokesman, the veteran Whitehall communications chief Mike Granatt, after he admitted that he had unwittingly misled the Mail on Sunday over £4,000 taxi expenses incurred by the speaker's wife, Mary. Granatt blamed unnamed officials, but not the speaker, for falsely informing him that the expenses were legitimate because Martin's wife had been accompanied by an official on shopping trips to buy food for receptions. It turned out that she had been accompanied by her housekeeper.

· Claims in two Sunday newspapers that Martin claimed £17,166 last year towards the cost of his Glasgow constituency home, on which he no longer pays a mortgage. The claim was made from the additional costs allowance, which is to help MPs who live away from London. David Davis, the shadow home secretary, said the speaker was in a difficult position. "Clearly he has got problems," Davis told BBC1's Andrew Marr programme.

MPs on both sides of the house hope that Martin, who will be 63 this year, will stand down as speaker before, or at least by, the next election in 2009 or 2010. A senior MP said: "I was astonished when the speaker announced last summer that he would stand at the next election. I had assumed that he would stand down in the autumn of this year. Betty Boothroyd stood down at that point in her term of office, which gave the Commons a chance to elect a new speaker before the next general election."

Well-placed figures believe that Martin, a proud man who believes that much of the criticism is fuelled by snobbery about his humble origins as a sheet metal worker, will be even more determined to stay on after the latest allegations. But some senior figures believe he may be prepared to make an announcement later this year if the criticisms die down - and after he has played a leading role at a meeting this summer of speakers from across the world. "That could be his swansong because he would play a leading role as the long-serving Speaker of the British parliament," an MP said.

Most MPs agree there is no appetite to move against Martin, even among his Tory critics, who believe he has little authority in the Commons and is biased in favour of Labour MPs. But Martin's position could change if he faces a prolonged investigation by the parliamentary standards commissioner. Lyon will move cautiously, because the TaxPayers' Alliance is a campaigning group which is highly effective at grabbing the headlines. But the alliance believes it is on strong ground over Mary Martin's taxi expenses for two reasons: she was not accompanied by an official and it is difficult to see how she could have been buying food for Commons functions when catering is provided by the parliamentary caterers.

Mark Wallace, the alliance's campaign director, said: "Michael Martin is charged with overseeing MPs' behaviour, so it is essential that his own behaviour is whiter than white." Martin faced further pressure when Sir Alistair Graham, the former standards watchdog, said he should no longer lead an inquiry into MPs' expenses. "What the latest story - and he has become the story - does suggest is that his review of MPs' expenses and allowance really cannot have any credibility now," Graham told the BBC.

David Blunkett, the former home secretary, told Sky News that Martin was facing a "witch-hunt". Margaret Beckett, the former foreign secretary, told the Andrew Marr programme: "There have been a whole string of nasty stories around Michael Martin. Clearly someone is out to get him. Whether any of it is valid, I can't judge."

FAQ

The rules

Has the Speaker broken any rules?

Not in any of the allegations levelled against him. There are no rules stopping official-business Airmiles being used to fund family flights. He can claim from the £23,000 additional costs allowance to fund his constituency home, regardless of whether he has a mortgage. Michael Martin may face queries on whether it was right for his wife, Mary, to claim more than £4,000 in taxi fares

Can the Speaker be sacked?

No. The Speaker enjoys a unique position based on having complete authority over the Commons

Is there a precedent for removal?

One Speaker in the 19th century was persuaded to stand down after showing a fondness for the bottle

Who is next in line to be Speaker?

Veteran Tory MP Sir George Young is frontrunner. Other candidates include Sylvia Heal, a deputy speaker from the Labour party